I wish most sincerely there was not a slave in this province. It always appeared a most iniquitous scheme to me — to fight ourselves for what we are daily robbing and plundering from those who have as good a right to freedom as we have.

Letter to John Adams (24 September 1774)

How difficult the task to quench the fire and the pride of private ambition, and to sacrifice ourselves and all our hopes and expectations to the public weal! How few have souls capable of so noble an undertaking! How often are the laurels worn by those who have had no share in earning them! But there is a future recompense of reward, to which the upright man looks, and which he will most assuredly obtain, provided he perseveres unto the end.

Letter to John Adams (10 July 1775)

I am more and more convinced that man is a dangerous creature; and that power, whether vested in many or a few, is ever grasping, and, like the grave, cries, “Give, give!” The great fish swallow up the small; and he who is most strenuous for the rights of the people, when vested with power, is as eager after the prerogatives of government. You tell me of degrees of perfection to which human nature is capable of arriving, and I believe it, but at the same time lament that our admiration should arise from the scarcity of the instances.

Letter to John Adams (27 November 1775)

The reins of government have been so long slackened, that I fear the people will not quietly submit to those restraints which are necessary for the peace and security of the community.

Letter to John Adams (27 November 1775)

I feel anxious for the fate of our monarchy, or democracy, or whatever is to take place. I soon get lost in a labyrinth of perplexities; but, whatever occurs, may justice and righteousness be the stability of our times, and order arise out of confusion. Great difficulties may be surmounted by patience and perseverance.

Letter to John Adams (27 November 1775)

I long to hear that you have declared an independency. And by the way, in the the new Code of Laws which I suppose it will be necessary for you to make I desire you would Remember the Ladies, and be more generous and favourable to them than your ancestors. Do not put such unlimited power into the hands of the Husbands. Remember all Men would be tyrants if they could. If particular care and attention is not paid to the Ladies we are determined to foment a Rebellion, and will not hold ourselves bound by any Laws in which we have no voice, or Representation.
That your Sex are Naturally Tyrannical is a Truth so thoroughly established as to admit of no dispute, but such of you as wish to be happy willingly give up the harsh title of Master for the more tender and endearing one of Friend. Why, then, not put it out of the power of the vicious and the Lawless to use us with cruelty and indignity with impunity? Men of Sense in all Ages abhor those customs which treat us only as the vassals of your sex; regard us then as Beings placed by Providence under your protection, and in imitation of the Supreme Being make use of that power only for our happiness.

Letter to John Adams (31 March 1776), published in Familiar Letters of John Adams and his wife Abigail Adams (1875) edited by Charles Francis Adams, p. 147

Shall we be despised by foreign powers for hesitating so long at a word?

Letter to John Adams (7 May 1776)

Whilst you are proclaiming peace and good will to men, emancipating all nations, you insist upon retaining an absolute power over wives. But you must remember that arbitrary power is like most other things which are very hard, very liable to be broken — and notwithstanding all your wise laws and maxims we have it in our power not only to free ourselves but to subdue our masters, and without violence throw both your natural and legal authority at our feet.

If you complain of neglect of Education in sons, what shall I say with regard to daughters, who every day experience the want of it? With regard to the Education of my own children, I find myself soon out of my depth, destitute and deficient in every part of Education.
I most sincerely wish that some more liberal plan might be laid and executed for the Benefit of the rising Generation, and that our new Constitution may be distinguished for encouraging Learning and Virtue. If we mean to have Heroes, Statesmen and Philosophers, we should have learned women. The world perhaps would laugh at me and accuse me of vanity, But you I know have a mind too enlarged and liberal to disregard the Sentiment. If much depends as is allowed upon the early education of youth and the first principles which are instill'd take the deepest root, great benefit must arise from literary accomplishments in women.

Letter to John Adams (14 August 1776)

It is really mortifying, sir, when a woman possessed of a common share of understanding considers the difference of education between the male and female sex, even in those families where education is attended to... Nay why should your sex wish for such a disparity in those whom they one day intend for companions and associates. Pardon me, sir, if I cannot help sometimes suspecting that this neglect arises in some measure from an ungenerous jealousy of rivals near the throne.

Letter to John Thaxter (15 February 1778)

I regret the narrow contracted education of the females of my own country.

Letter to John Adams (30 June 1778)

If we do not lay out ourselves in the service of mankind whom should we serve?

Letter to John Thaxter (29 September 1778)

Luxury, that baneful poison, has unstrung and enfeebled her sons.

Letter to John Adams (13 February 1779)

These are times in which a genius would wish to live. It is not in the still calm of life, or in the repose of a pacific station, that great characters are formed. The habits of a vigorous mind are formed in contending with difficulties. Great necessities call out great virtues. When a mind is raised, and animated by the scenes that engage the heart, then those qualities which would otherwise lay dormant, wake into life and form the character of the hero and the statesman.

Patriotism in the female sex is the most disinterested of all virtues. Excluded from honors and from offices, we cannot attach ourselves to the State or Government from having held a place of eminence. Even in the freest countries our property is subject to the control and disposal of our partners, to whom the laws have given a sovereign authority. Deprived of a voice in legislation, obliged to submit to those laws which are imposed upon us, is it not sufficient to make us indifferent to the public welfare? Yet all history and every age exhibit instances of patriotic virtue in the female sex; which considering our situation equals the most heroic of yours.

Letter to John Adams (17 June 1782)

I begin to think, that a calm is not desirable in any situation in life. Every object is beautiful in motion; a ship under sail, trees gently agitated with the wind, and a fine woman dancing, are three instances in point. Man was made for action and for bustle too, I believe.

Letter to her sister, Mary Smith Cranch (1784)

To be good, and to do good, is the whole duty of man comprised in a few words.

Letter to Elizabeth Shaw (1784), quoted in John Adams (2001) by David McCullough, p. 310

Knowledge is a fine thing, and mother Eve thought so; but she smarted so severly for hers, that most of her daughters have been afraid of it since.

Letter to Elizabeth Shaw (20 March 1791)

I acknowledge myself a unitarian — Believing that the Father alone, is the supreme God, and that JesusChrist derived his Being, and all his powers and honors from the Father. … There is not any reasoning which can convince me, contrary to my senses, that three is one, and one three.

This seems to first appear in Why Leaders Can't Lead : The Unconscious Conspiracy Continues (1989) by Warren G. Bennis, p. 159, where it is cited as being from a letter to Thomas Jefferson, but it might be a misquote of "Great necessities call out great virtues" stated in a letter to her son John Quincy Adams (19 January 1780)